During semiconductor assembly processes, there is sometimes a necessity to make electrical connections between connection pads of electronic devices. The electrical connections can be formed by using conductive wires to establish linkages between the connection pads. The most widely used wire materials are Gold (Au) and Aluminum (Al), but Silver (Ag) and Copper (Cu) are also used. The connection pads may comprise metallized bond sites on a semiconductor chip or on interconnection substrates. A wire bond secures the wire to the connection pad in order to ensure that the electrical connection is secure and the wire is not easily dislodged from the connection pad.
There is typically a first bonding position where a first wire bond is made and a second bonding position where a second wire bond is made. Generally, the wire bonding process involves feeding a conductive wire through a capillary of a wire bonding device and using the capillary to manipulate and bond the wire.
A typical wire bonding process is hereinafter described. The capillary is first located over a first bonding position. A clamp controlling the wire opens and wire extends out of the capillary. An electro flame-off spark is generated to create a free air ball at a tail of the wire and the capillary moves towards the first bonding position with the free air ball. The free air ball is placed onto the first bonding position, and ultrasonic energy and pressure is applied onto the ball to create a first wire bond between the wire and the connection pad at the first bonding position.
After the first bond is made, the capillary moves away from the first bonding position and wire is extended by the capillary as the capillary is moved towards the second bonding position in order to form a wire loop. The capillary moves to the second bonding position and presses the wire onto second bonding position. Ultrasonic energy and pressure is applied onto the wire and stitch bonding is performed to the wire at the capillary tip, thereby stitching the wire to the connection pad at the second bonding position. After the second bond is made, the capillary moves away from the connection pad at which point the wire has been bonded between two points. As the capillary moves away from the second bonding position, the clamp is closed such that the wire is pulled and severed from the wire bond made at the second bonding position.
It may be difficult to form a sufficiently strong stitch bond at the second bonding position for certain types of connection pads using conventional stitch bonding. The stitch pull tolerance of the wire bond, which is the amount of pulling force the wire bond can withstand before dislocation, is usually small and the stitch quality is not good, especially when conventional stitch bonding is utilized for ultra-fine pitch wire bonding. If the bond is not strong, the wire may be easily pulled away from the connection pad, leading to unreliability of the electrical connection made. In order to increase the bond strength, special bonding techniques were developed, such as the so-called ball bond on stitching (“BBOS”) or ball stitching on bond (“BSOB”).
BBOS involves placing a ball bump on top of the stitch bond already made at the second bonding position. The process is described in U.S. Pat. No. 5,960,262 entitled “Stitch Bond Enhancement for Hard-to-Bond Materials”. On the other hand, BSOB involves first forming a ball bump at the second bonding position, before placing a stitch bond on top of the ball bump. This process is described in U.S. Pat. No. 5,328,079 entitled “Method and Arrangement for Bond Wire Connecting Together Certain Integrated Circuit Components”. Both these methods assist in increasing a contact area between a wire bond and a connection pad so as to increase their degree of intermetallization.
However, the said BBOS and BSOB techniques present some problems. Although they serve to improve the strength of the second stitch, they may cause sway wire or snake wire problems due to the construction of the bonds. Sway wire refers to a tendency for the wire loop to deviate from a straight line passing through the connection pads at the first and second bonding positions and therefore threaten to contact adjacent wire loops, which might cause a short-circuit. This is sometimes due to the fact that the ball bump does not sufficiently anchor the wire to prevent sideways motion. Snake wire refers to a tendency of the wire in the capillary to recoil after a wire is broken at the second bond position when forming a ball bump, thus affecting the linearity of the wire for the next bond. There is at present no effective way to solve these problems with the BBOS and BSOB techniques. Furthermore, the BBOS and BSOB techniques require a longer bonding cycle time because of the need to form a ball bump in addition to forming a stitch bond.